Referring to FIG. 1, a wireless sensor network 1 is formed of a sensor field 300 where a plurality of sensor nodes 100 are disposed, and a sink node 400 connecting the sensor field 300 to a base station 500. The sensor nodes 100 of the sensor field 300 acquire sensing data, which are transmitted to the base station 500 through the sink node 400. The wireless sensor network 1 is used in diverse areas such as health monitoring, military networks, home networks, environmental observation, factory management, and anti-disaster management.
Each of the sensor nodes 100 has their own sensor node duration set up in consideration of a power source and lifespan (i.e., device duration) of respective devices constituting the sensor node. The sensor node duration will be referred to as ‘energy,’ hereinafter. The energy of each sensor node 100 is limited and it cannot be recharged.
Particularly, the energy of a sensor node 100 is rapidly drained when data are transmitted/received wirelessly in the wireless sensor network 1.ETx(k,d)=(k×Eelec)+(k×εamp×d2)  Eq. 1
where ETx denotes the total amount of energy consumption during data transmission; k denotes the amount of data packets; d denotes wireless transmission/reception range; εamp denotes a constant of the amount of energy consumed during signal amplification in an amplifier; and Eelec denotes a constant of the amount of energy consumed when data are converted into radio frequency (RF).ERx(k,d)=k×Eelec  Eq. 2
where ERx denotes the total amount of energy consumption during data reception; k denotes the amount of data packets; and Eelec denotes a constant of the amount of energy consumed when data received in the form of RF are converted.
As shown in FIG. 2 and Equation 1, energy consumption increases in proportion to the amount (k) of packets and squared range during data transmission. During data reception, the energy consumption increases in proportion to the amount (k) of packets, as shown in FIG. 2 and Equation 2.
Therefore, it is desirable to reduce the number of times of performing wireless communication as much as possible in order to save energy of the sensor nodes 100.
Among methods for operating the wireless sensor network 1 is a Low Energy Adaptive Clustering Hierarchy (LEACH). LEACH is a method that sensor nodes 100 form a cluster 200 and a cluster head 110 randomly selected collects all informations in the cluster 200 and transmits them to the base station 500 through the sink node 400.
LEACH is executed through a selection stage, a cluster setup stage, a schedule generation stage, and a data transmission stage.
                              P          ⁡                      (            n            )                          =                  {                                                                      k                                      1                    -                                          k                      ×                                              (                                                  r                          ⁢                                                                                                          ⁢                                                      mod                                                    ⁢                                                                                                          ⁢                                                      1                            k                                                                          )                                                                                                                                                              if                    ⁢                                                                                  ⁢                    n                                    ∈                                      C                    ⁡                                          (                      n                      )                                                                                                                          0                                            otherwise                                                                        Eq        .                                  ⁢        3            
where P(n) denotes a threshold value; k denotes a constant indicating a composition ratio of a cluster head; r denotes a current round (i.e., operation time unit of LEACH); C(n) denotes a group of nodes which are not cluster heads for a past 1/k round.
In the cluster head selection stage, a threshold P(n) acquired from the Equation 3 is compared with a random number between 0 and 1 that is randomly selected. A sensor node 100 whose threshold P(n) is greater than the random number is selected as the cluster header 110. In a round, each sensor node 100 can be a cluster head 110 at a k probability. In the round, a sensor node 100 that becomes the cluster head 110 cannot be a cluster head 110 again for the next 1/k round. As rounds continue, an even smaller number of sensor nodes 100 are qualified to be selected as the cluster head 110, the probability of the remaining sensor nodes 100 to be selected as the cluster head 110 increases.
In the cluster setup stage, selected cluster heads 110 transmit an advertisement message to the sensor nodes 100. The sensor nodes 100 that have received the advertisement message from a plurality of cluster heads select a cluster head 110 to join based on received signal strength (RSS). A sensor node 100 transmits a join-request message (Join-REQ) to the cluster head 110 to join and set up a cluster 200.
In the scheduling setup stage, the cluster head 110 performs Time Division Multiple Access (TDMA) to allocate time to the sensor node 100 that has sent out the join-request message to itself. Accordingly, each sensor node 100 transmits a sensing result to the cluster head 110 at an allocated time.
In the data transmission stage, the sensing results transmitted from the sensor nodes 100 at the allocated time are collected, and data fusion is performed. The fused data are transmitted to the base station 500 through the sink node 400.
As described above, LEACH has an effect of extending the lifespan of the wireless sensor network 1 by randomly selecting cluster heads 110 in charge of transmitting data to the base station 500 and thereby distributing energy consumed for data transmission to all sensor nodes 100.
However, since LEACH randomly selects the cluster heads 110, even a sensor node 100 with a relatively smaller amount of residual energy than those of other sensor nodes 100 can be selected as a cluster head 110. Since the cluster head 110 should perform data fusion onto sensing results transmitted from sensor nodes 100 in a cluster 200 and deliver the fused data to the base station 500 through the sink node 400, it consumes more power than other sensor nodes 100.
Therefore, when a sensor node 100 with a relatively smaller amount of residual energy than those of other sensor nodes 100 is selected as a cluster head 110, it may consume all its energy for excessive data transmission and eventually complete its own lifespan. Consequently, there is a shortcoming in that when a sensor node 100 with a relatively smaller amount of residual energy than those of other sensor nodes 100 is selected as a cluster head 110, the lifespan of the wireless sensor network 1 is shortened.
To overcome this shortcoming, LEACH-C is developed. LEACH-C is a method of positioning the base station 500, the sink node 400, and the sensor nodes 100 and selecting cluster heads 110 in consideration of residual energy amount of each sensor node 100. In other words, since cluster heads 110 are selected in consideration of the entire residual energy amount of the wireless sensor network 1, there is no concern in the LEACH-C that a sensor node 100 with a small amount of residual energy might be selected as a cluster head 110.
Also, to figure out the positions and residual energy amounts of the base station 500, the sink node 400, and the sensor nodes 100 in LEACH-C, data on the positions and residual energy amounts should be transmitted/received among the sensor nodes 100. This also causes severe energy consumption during the data transmission/reception.